This is not possible via a list import in Marketo. You will have to either do it in your CRM or create a smart campaign that updates the records' owner. This can be time consuming if there are multiple owners in your list.

There were two different lead owner's in one column. So, I just separated the list like normal and ran the change owner step in the flow. Just thought there might have been another way since they have the owner field available in the drop down.